(9) باب كَرَاهَةِ الشُّرُوعِ فِي نَافِلَةٍ بَعْدَ شُرُوعِ الْمُؤَذِّنِ
Abu Huraira reported the Messenger of Allah (ﷺ) as saying:
When the prayer commences, there is no prayer but the obligatory one. (ID)
Reference (ID): Sahih Muslim 710 b (ID)
In-book reference (ID): Book 6, Hadith 76 (ID)
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1532 (ID)