Allatī is a feminine relative pronoun referring to an understood jamrah. Talī is an imperfect verb meaning 'lies next to' or 'is near'; the object al-masjida identifies the mosque.
Bi-sabʿi ḥaṣayāt is the numerical phrase 'with seven pebbles'. Yukabbiru is the Form II imperfect of k-b-r, used here for pronouncing the takbir.