An Open Supererogatory Prayer

Question :

Is it permissible to offer a supererogatory prayer with an open intention. For example, a man performed a supererogatory prayer and he did not determine how many Rak'ahs he intended to pray, desiring to pray as many Rak'ahs as Allah willed. Is this permissible, and what is the evidence? And if he made the Taslim for this prayer, and he did not know if he had prayed an odd or an even number, nor how many Rak'ahs he had prayed, what would be the ruling?

Answer:

The ruling on this is that a person is not free to choose whatever number of Rak'ahs he wishes, because he is bound by what has been set forth by the Islamic law; and the Prophet said:
night prayer and the day prayer are two by two...Abu Dawud no. 1295

"The Therefore, it is prescribed for a person to pray two by two during the night and the day, and he should not join four, six or eight or the like together.

The exception to this is that it has been written that it is permissible to perform three Rak'ahs of Witr together, with one Tashahhud; likewise, the Sunnah when performing five is to offer them with one Taslim and one Tashahhud, and also when praying seven; and when offering nine, with one Taslim and two Tashahhuds. The first following the eighth Rak'ah and the second, along with the Taslim, following the ninth. So a person is not free to choose to pray however many Rak'ahs he likes. As for praying whatever (supererogatory) prayers he wishes, there is no sin upon him in that; he may pray whatever prayers he wishes, although it is better at night not to pray more than thirteen or eleven Rak'ahs, as the Messenger used to do.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Pages 321-322