344 - Chapter on the dislike of embarking upon a supererogatory prayer after the Muezzin has begun to proclaim the Iqāmah for prayer, regardless of whether the supererogatory prayer is the Sunnah of this very prayer or otherwise

1759/1 - Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah’s peace and blessings be upon him) said: “When Iqāmah (second call for prayer) is pronounced, there is no prayer to be offered but the obligatory one.” [Narrated by Muslim]

Guidance from the Hadīth:

1) It is not permissible to embark upon or complete a supererogatory prayer after Iqāmah has been pronounced for an obligatory one, for the obligatory takes precedence over the voluntary.

2) A person with proper understanding would prioritize acts of worship if they came at the same time.

Benefit:

If Iqāmah for prayer is proclaimed while you are in a supererogatory prayer, cut the prayer if you are still in the first Rak‘ah (unit of prayer). But if you are in the latter part of the prayer, like Tashahhud, then complete it quickly. To cut the prayer, get out of it without Taslīm (salutation of peace ending the prayer), for Taslīm is to be made after Tashahhud, and Allah knows best.