On hearing Athan - 1
لَا حَوْلَ وَلَا قُوَّةَ إِلَّا بِاللهِ
Laa hawla wa laa quwwata 'illaa billaah
There is no might and no power except by Allah.
Al-Bukhari 1:152
Supplication
On hearing Athan
2 duas with Arabic, transliteration and translation.
On hearing Athan - 2
اللَّهُمَّ رَبَّ هَذِهِ الدَّعْوَةِ التَّامَّةِ، وَالصَّلَاةِ الْقَائِمَةِ، آتِ مُحَمَّداً الْوَسِيلَةَ وَالْفَضِيلَةَ، وَابْعَثْهُ مَقَاماً مَحْمُوداً الَّذِي وَعَدْتَهُ، إَنَّكَ لَا تُخْلِفُ الْمِيعَادَ.
Allaahumma Rabba haathid - da'watit - taammati wassalaatil-qaa'imati, 'aati Muhammadanil-waseelata walfadheelata, wab 'ath-hu maqaamam-mahmoodanil-lathee wa'adtahu, 'innaka laa tukhliful-mee'aad
O Allah , Lord of this perfect call and established prayer. Grant Muhammad the intercession and favor, and raise him to the honored station You have promised him, [verily You do not neglect promises].
Al-Bukhari 1:152Each dua is shown with its reference where the source records one, so you can check it. Duas whose wording you intend to rely on in worship are worth confirming with a qualified teacher.