Musnad Abi Ya'la 705 — I asked my father, Sayyiduna Sa'd: Father!

Mus'ab bin Sa'd (may Allah have mercy on him) narrates: I asked my father, Sayyiduna Sa'd (may Allah be pleased with him): "Father! 'Those who are heedless of their prayer': does it mean one of us thinking to himself during his prayer?" Sayyiduna Sa'd (may Allah be pleased with him) said: "Do we not all do that? Rather, the one heedless of his prayer is the one who prays it outside its time; that is the one heedless of it." On another occasion Mus'ab (may Allah have mercy on him) put it this way: "(It means) not performing the prayer at its appointed times."