Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: I remember about ourselves (that is, the Companions) that (in the blessed time of the Prophet (peace and blessings be upon him)) the prayer would not begin until the rows were complete.
Imam Ahmad recorded it, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2494
Hadith Gradingمحدثین:إسنادہ صحیح۔
Hadith Takhrij«مسند احمد بن حنبل ، مسند عبد الله بن مسعود رضي الله تعالي عنه 3861 ، أخرجه مسند أحمد برقم: 3969»