From Sahih Muslim (Hadith 252):
It is narrated on the authority of 'Abdullah b. Mas'ud that he observed. I asked the Messenger of Allah (ﷺ) which deed was the best. He (the Holy Prophet) replied:Prayer at its appointed hour. I (again) said: Then what? He (the Holy Prophet) replied: Kindness to the parents. I (again) said: Then what? He replied: Earnest endeavour (Jihad) in the cause of Allah. And I would have not ceased asking more questions but out of regard (for his feelings)
Reference: Muslim 252