What is the ruling on saying "Astaghfirullah (I seek forgiveness from Allah)" in the mosque when the Imam finishes reciting Al-Fatiha?

There is no evidence for the permissibility of seeking forgiveness after reciting Al-Fatiha. The Malikis permitted supplication after Al-Fatiha. Al-Khattab, in "Mawahib Al-Jaleel," transmitted the saying of the author of "Al-Tiraz": "And he supplicates after finishing Al-Fatiha if he wishes, before the surah, and the righteous have indeed supplicated."