Imam Ahmad said:

Sincerity is when your Actions – from worship, avoiding prohibitions, and [performing] every righteousness and piety – are solely intended for Allah, Glorified is He.

Sincerity is the Soul of the Deeds, and Deeds without their Soul are dead, so neither does Allah accept it, nor will it be a saviour from the Hellfire [1].

[1] Ahmad ibn Hanbal, by ‘Abd al-Ghani Ad-Daqr, in the Series of A’laam al-Muslimeen, p. 268, transmitted by Ibn ‘Asaakir.

قال الإمام أحمد: الإخلاص: أن يكون عملك – من عبادة, واجتناب محرمات, وكل بر وتقوى – خالصا بالقصد لله سبحانه. والإخلاص روح العمل, والعمل بغير روح عمل ميت فلا الله يقبله, ولا هو بمنج من النار.ـ