Muhammad ibn Yahya narrated from Ahmad ibn Muhammad from Al-Husayn ibn Sa'id from Uthman ibn Isa from Ishaq ibn Ammar who said: I asked: “May I pray a supererogatory prayer during the time of an obligatory prayer?” He said: "Yes, at the beginning of the time, if you are with an Imam whom you follow. But if you are alone, then start with the obligatory prayer."