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 Messenger of Allah ﷺ prayed when the sun was eclipsed, bowing eight times and prostrating four times. (Using translation from Nasāʾī 1467)   

أَنَّ رَسُولَ اللهِ ﷺ صَلَّى عِنْدَ كُسُوفِ الشَّمْسِثَمَانِيَ رَكَعَاتٍ وَأَرْبَعَ سَجَدَاتٍ  

[Machine] "That the Messenger of Allah ﷺ prayed eight rak'ahs and four prostrations during a solar eclipse, and Ata' narrated the same to us, saying: 'Abu Abdur Rahman said: This is a good hadith.'"  

أَنَّ رَسُولَ اللهِ ﷺ صَلَّى عِنْدَ كُسُوفٍ ثَمَانِيَ رَكَعَاتٍ وَأَرْبَعَ سَجَدَاتٍ وَعَنْ عَطَاءٍ مِثْلُ ذَلِكَ قَالَ لَنَا أَبُو عَبْدِ الرَّحْمَنِ هَذَا حَدِيثٌ جَيِّدٌ  

The Messenger of Allah ﷺ prayed when the sun was eclipsed, bowing eight times and prostrating four times. (Using translation from Nasāʾī 1467)  

أَنَّ رَسُولَ اللهِ ﷺ «صَلَّى عِنْدَ كُسُوفٍ ثَمَانِيَ رَكَعَاتٍ وَأَرْبَعَ سَجَدَاتٍ» وَعَنْ عَطَاءٍ مِثْلَ ذَلِكَ