عَنْ أَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: «خَرَجَ رَسُولُ اَللَّهِ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ إِلَى اَلصَّلاَةِ وَ قَدْ كَانَ اَلْحُسَيْنُ عَلَيْهِ اَلسَّلاَمُ أَبْطَأَ عَنِ اَلْكَلاَمِ حَتَّى تَخَوَّفُوا أَنَّهُ لاَ يَتَكَلَّمُ وَ أَنْ يَكُونَ بِهِ خَرَسٌ فَخَرَجَ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ بِهِ حَامِلاً عَلَى عَاتِقِهِ وَ صَفَّ اَلنَّاسُ خَلْفَهُ فَأَقَامَهُ عَلَى يَمِينِهِ فَافْتَتَحَ رَسُولُ اَللَّهِ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ اَلصَّلاَةَ فَكَبَّرَ اَلْحُسَيْنُ عَلَيْهِ اَلسَّلاَمُ فَلَمَّا سَمِعَ رَسُولُ اَللَّهِ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ تَكْبِيرَهُ عَادَ فَكَبَّرَ وَ كَبَّرَ اَلْحُسَيْنُ عَلَيْهِ اَلسَّلاَمُ حَتَّى كَبَّرَ رَسُولُ اَللَّهِ صَلَّى اَللَّهُ عَلَيْهِ وَ آلِهِ سَبْعَ تَكْبِيرَاتٍ وَ كَبَّرَ اَلْحُسَيْنُ عَلَيْهِ اَلسَّلاَمُ فَجَرَتِ اَلسُّنَّةُ بِذَلِكَ.
Imam Muhammad ibn Ali Al-Baqir (a.s) narrated that He said: "The Messenger of Allah (saws) went out for prayer while carrying Imam al-Husayn (a.s) on his shoulder, as al-Husayn (a.s) had delayed in speaking, and there was concern that he might have a speech impediment or be unable to speak. The Prophet (saws) carried him out, and the people lined up behind him (for prayer). He placed al-Husayn (a.s) on his right side. The Prophet (saws) began the prayer, and when He said the takbir (saying 'Allahu Akbar'), Imam al-Husayn (a.s) repeated the takbir after him. When the Prophet heard al-Hussayn’s takbir, he repeated it, and so did al-Husayn. This continued until the Prophet (saws) had said seven takbirs, with al-Husayn repeating them. Thus, the tradition (of seven takbirs at the beginning of the prayer) was established."