Hadith.963 - Zurarah reported that he heard Abu Ja'far (as) say:
"Supplication after the obligatory prayer is better than offering a voluntary prayer."
And thus, this has been the practice.
Al-Kāfi #1305“I heard abu ‘Abd Allah (a.s.), say, ‘The excellence of prayer, after an obligatory Salah (prayer), over prayers after non-obligatory Salah...