When there was a solar eclipse the Messenger of Allah (way peace be upon him) observed eight ruku's and four prostrations (in two rak'ahs). This has been narrated by 'Ali also.  

صَلَّى رَسُولُ اللَّهِ ﷺ حِينَ كَسَفَتِ الشَّمْسُ ثَمَانَ رَكَعَاتٍ فِي أَرْبَعِ سَجَدَاتٍ وَعَنْ عَلِيٍّ مِثْلُ ذَلِكَ  

The Apostle of Allah ﷺ observed prayer while it was (solar) eclipse. He recited (the Qur'an in qiyam) and then bowed. He again recited and again bowed. He again recited and again bowed and again recited and again bowed, and then prostrated; and the second (rak'ah) was like this.  

عَنِ النَّبِيِّ ﷺ أَنَّهُ صَلَّى فِي كُسُوفٍ قَرَأَ ثُمَّ رَكَعَ ثُمَّ قَرَأَ ثُمَّ رَكَعَ ثُمَّ قَرَأَ ثُمَّ رَكَعَ ثُمَّ قَرَأَ ثُمَّ رَكَعَ ثُمَّ سَجَدَ قَالَ وَالأُخْرَى مِثْلُهَا