Valmiki Ramayana Ayodhya Kanda Sarga 110 Shloka 7
Original Shlokaयस्येयं प्रथमं दत्ता समृद्धा मनुना मही। तमिक्ष्वाकुमयोध्यायां राजानं विद्धि पूर्वकम्।।2.110.7।।
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यस्य to whom, प्रथमम् originally, समृद्धा prosperous, मही earth, मनुना by Manu, दत्ता was bestowed, तम् that, इक्ष्वाकुम् Ikshvaku, अयोध्यायाम् of Ayodhya, पूर्वकम् first, राजानम् king, विद्धि please know.
Manu had orginally bestowed the most prosperous earth on Ikshvaku and you know Ikshvaku was the first king of Ayodhya.