Muhammad ibn Yahya narrated from Muhammad ibn Al-Husayn from Uthman ibn Isa from Sama'ah who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who comes to the mosque and the people there have already prayed. Should he start with the obligatory prayer or with a supererogatory prayer? Imam عليه السلام said: "If it is within a good time, then there is no harm in performing the supererogatory prayer before the obligatory one. But if he fears missing the obligatory prayer because a lot of time has passed, then he should start with the obligatory prayer, as it is the right of Allah سبحانه وتعالى. Then he may pray as many supererogatory prayers as he wishes. The matter is flexible; a person may pray at the beginning of the obligatory prayer’s time. And it is better, if a person is praying alone, to begin with the obligatory prayer once its time has entered, so that the best part of the time is devoted to the obligatory prayer. However, it is not prohibited for him to perform supererogatory prayers from the beginning of the time until near the end of the time."