It is narrated from Sayyiduna Samurah (may Allah be pleased with him); he says that the Messenger of Allah (peace and blessings be upon him) used to order us that if one of us is kept busy from a prayer, or forgets it until its time in which it is prayed has passed, we should pray it along with the obligatory prayer that follows it.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 7004