Is it preferable to offer Al-Sunan

Q 2: Some worshipers interrupt the supererogatory Salah (Prayers) they are performing once the obligatory Salah commences; is this permissible?

(Part No. 7; Page No. 240)  A: Once the obligatory Salah commences, it is not permissible to start offering a supererogatory Salah. This is based on the general ruling in the Hadith of the Prophet (peace be upon him): When Iqamah is announced, there should be no Salah except the obligatory one. (Related by Muslim and others) Accordingly, when the obligatory Salah commences while a worshiper is offering a supererogatory one, they should stop it in compliance with the Hadith mentioned previously; also because the obligatory Salah takes precedence.