Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: When the prayer has begun (that is, when you have started praying), do not leave it for something else.
Al-Tabarani recorded it, and its narrators are thiqat (trustworthy), except that the narrator Ziyad did not hear from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2579