What is the ruling on an individual praying alone when there is a congregation, or praying alone after having started the prayer and then a congregation forms, or praying alone due to an error the worshipper perceives on the part of the Imam? And what is the ruling on praying behind an Imam who says "Allahu Akbar" after fully standing up from prostration, or who says "Sami'a Allahu liman hamidah" after fully standing up from bowing?

Congregational prayer is, according to the preponderant opinion, an individual obligation (fard ayn) upon free, adult, non-excused men. Whoever intentionally prays alone without an excuse after the congregational prayer has been established is sinful. If a worshipper begins a fard (obligatory) prayer before the iqamah (call to commence prayer) and then the iqamah is made, it is preferable for him to complete it as a supererogatory prayer (nafl) to attain the reward of the congregation.

If the Imam makes an error that invalidates the prayer, such as omitting a pillar, then it is not permissible to pray behind him. However, if the error does not invalidate the prayer, like a subtle mispronunciation (lahn khafi), it does not cause harm and does not prevent the validity of following him. It is permissible for people of different madhhabs (schools of thought) to pray behind each other.

The place for the takbirat al-intiqal (takbirs for moving between positions) and tasmi’ (saying “Sami’a Allahu liman hamidah”) is during the transition to the next pillar. According to the majority of scholars, abandoning them or performing them in the wrong place does not invalidate the prayer because they are sunnah (recommended). However, according to the Hanbalis, the madhhab maintains the obligation of the takbirat al-intiqal and tasmi’. Whoever performs them intentionally after the transition, their prayer is invalid. Some verifying scholars have, however, deemed that it does not invalidate the prayer. Whoever omits them due to forgetfulness or ignorance should perform the prostration of forgetfulness (sujud al-sahw), and their prayer is not invalidated.

One should not abandon praying behind an Imam who performs the takbir or tasmi’ in the wrong place. Rather, the Sunnah regarding this should be gently explained to him.