In The Name of Allaah, The Most Merciful, The Bestower of Mercy

It was reported from Abu ‘Abdur-Rahmaan Al-Hubli, from As-SunãbihI, from Mu’aadh bin Jabal, that the Messenger of Allah [sallal-laahu-alayhi-wasallam] held his hand and said: “O Mu’aadh! I swear by Allah, I love you. I swear by Allah, I love you. I advise you, O Mu’aadh, that you never leave saying after every prayer:

[اللَّهُمَّ أَعِنِّي عَلَى ذِكْرِكَ وَشُكْرِكَ وَحُسْنِ عِبَادَتِكَ – O Allah! Help me in remembering You, thanking You, and perfecting my worship of You].” And Mu’aadh advised As-Sunaabihi with that, and As-Sunaabihi advised Abu ‘Abdur-Rahmaan with that. [Ref 1]

Imaam Abdul-Azeez Bin Baaz [rahimahullaah] says that it is better to supplicate with this supplication after the Tasha’hud and before the Tasleem, but there is no harm in doing so after the Tasleem. For further details, refer to this link to find out why the Shaikh (rahimahullaah) says so.  http://www.binbaz.org.sa/node/949

[Ref 1] Declared Saheeh [authentic] by Imaam Albaani (rahimahullaah) in his checking of Sunan Abee Dawud Hadeeth number: 1522