"Whoever prays twelve rak'ah during the night and day, a house will be built for him in Paradise: four before Dhuhr and two after, two rak'ah after Maghrib, two rak'ahs after 'Ishā' and two rak’ah before Fajr prayer." [an-Nasā'ī: 1794]

These are prayers that the Prophet (may the peace and blessings of Allah be upon him) used to pray, but he would not be as regular in performing them as those mentioned above.

The Ḍuḥaa Prayer

The most important of all of the remaining voluntary prayers is the Ḍuḥaa prayer. Abu Dharr (may Allah be pleased with him)reported that the Prophet (may the peace and blessings of Allah be upon him) said:

"In the morning, charity is due on every joint bone of the body of everyone of you. Every utterance of subḥānallāh is an act of charity, and every utterance of al-ḥamdulillāh is an act of charity and every utterance of laa ilaaha il-Allāh is an act of charity; and enjoining of good is an act of charity, and forbidding is an act of charity, and two rak'ah of the Ḍuḥaa prayers which one performs in the forenoon is equal to all this (in reward)." [Muslim: 720]

As for the time of ḍuḥaa, it is from the moment that the sun has completely risen over the horizon, until the sun reaches its zenith. However, it is recommended to pray it after half of the morning has passed, because the Prophet (may the peace and blessings of Allaah be upon him) said:

"It is preferable to offer ḍuḥaa prayer when the sun’s heat has become so intense that even the young of the camels feel it." [Muslim: 748]

As for the number of rak'ah, it is prayed in prayers of two, and it is preferred not to exceed eight rak'ah in total.

There is also a particular form of praying the ḍuḥaa prayer which excels in reward, and that is to remain in the mosque after Fajr remembering Allah, and then, when the sun has completely risen over the horizon, to pray two rak'ah. The Prophet (may the peace and blessings of Allah be upon him) said:

"Whoever prays the Fajr prayer in congregation then sits remembering Allah until the sun rises, then prays two units of prayer has the reward like that of Hajj and 'Umrah [the greater and lesser pilgrimages]: entirely, entirely, entirely." [at-Tirmidhī: 586]

Optional Prayers Around the Obligatory Prayers

Just as we mentioned the regular voluntary prayers that are timed around the obligatory prayers, there are other optional prayers which are similar, except that they do not have a specific reward for them:

Two sets of two rak'ah before the 'Aṣr prayer. The Prophet (may the peace and blessings of Allah be upon him): "May Allah have mercy on someone who prays before Aṣr four [rak'ah]." [Abū Dāwood: 1271]
Two rak'ah before Maghrib. Abūl-Khayr narrated that "Abū Tameem al-Jayshāni stood up to pray two rak'ahs before Maghrib, and I said to 'Uqbah bin 'Amir: 'Look at this man, what prayer is he praying?' He turned and looked at him, and said: 'This is a prayer that we used to pray at the time of the Messenger of Allah (may the peace and blessings of Allah be upon him)'." [an-Nasā'ī: 582]
Two rak'ah before 'Ishā', because the Prophet (may the peace and blessings of Allah) said, "Between every two calls to prayer (i.e., the adhaan and the iqaamah) there is a prayer [he repeated it three times], for those who wish." [al-Bukhārī: 624]

Prayers Which Have a Cause

There are a number of other prayers which are triggered by certain actions, including:

Two rak'ah after wuḍoo', because the Prophet (may the peace and blessings of Allah be upon him) said: "Whoever makes wuḍoo', like this wuḍoo' of mine, then prays two rak'ah, without any thoughts crossing their mind, except that Allah forgives them that which has preceded of their sin." [al-Bukhārī: 159]
The prayer of greeting the masjid, which is that before a person sits upon entering the masjid, they pray two rak'ah (or they join a congregational prayer), because the Prophet (may the peace and blessings of Allah be upon him) said: "When anyone among you enters the mosque, he should not sit till he has observed two rak'ah" [Muslim: 714]. The stronger opinion is that this prayer is obligatory, because the Prophet (may the peace and blessings of Allah be upon him) did not even make an exception during the Friday sermon for the one who came late.

Other Prayers

Other prayers include:

The night prayer, which is a set of two rak'ah prayers, and can be prayed from the end of the 'Ishaa' congregational prayer until the beginning of the time for Fajr. The best time to pray this is the last third of the night (as calculated as the time between Maghrib and Fajr, divided by three), because the Prophet (may the peace and blessings of Allah be upon him) said, "Our Lord, the Blessed and the Exalted, descends every night to the lowest heaven when one-third of the latter part of the night is left, and says: Who supplicates Me so that I may answer him? Who asks Me so that I may give to him? Who asks Me forgiveness so that I may forgive him?" [Muslim: 758] He also said: "Prayer at night is two by two, and witr is one rak'ah." [an-Nasā'ī: 1693], and he would not pray more than eight rak'ah, excluding the witr prayer, because of the narration of Aa'ishah (may Allah be pleased with her): "Allah's Messenger (may the peace and blessings of Allah be upon him) never exceeded eleven rak'ah in Ramaḍān or in other months..." [al-Bukhārī: 1147]
The witr prayer.
The prayer for seeking guidance (istikhaarah).