As per Al Harith Bn Yazid he heard Rabiaa Al Garshi saying: That the Prophet (PBUH) said: “Stand upright, the best is if you stood upright. And preserve your ablution as the best of your deeds is prayer”
Al-Muʿjam al-Kabīr of al-Tabarani (4596), classed as saheeh by al-Albaani in Saheeh al-Jaami '(953).
(Stand upright, and the best is if you stood upright) as to be upright has a great and significant effect, and then it was said that no verse was greater or tougher than this verse: “So be upright as you were commanded”
Ibn al Athir said: I was told that the greatest pillar of being upright is prayer, as per his saying: “And your best deed is prayer”.
(And preserve your ablution) by mastering it, performing it regularly and applying all its Sunnahs and teachings.
Fayd Al Kadir (1/497)