It is narrated from Nafi' that Sayyiduna Ibn Umar (may Allah be pleased with them both) would sometimes give the adhan for the morning prayer while seated on his mount, then give the iqamah standing on the ground.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1841
It is narrated from al-Hasan (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) gave Sayyiduna Bilal (may Allah be pleased with him) an order during a journey, and he gave the adhan on his mount. Then they dismounted and prayed two rak'ahs; then he was ordered and gave the iqamah, and he (peace and blessings be upon him) led them in the morning prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1843
(a) It is narrated from al-Hasan bin Muhammad: I went to Abu Zayd al-Ansari (may Allah be pleased with him); he gave the adhan and the iqamah while he was seated. A man stepped forward and led us in prayer; he was lame, his leg having been injured in the path of Allah.
(b) It is reported from Ata' bin Abi Rabah (may Allah have mercy on him) that he disliked that the adhan be given sitting without an excuse.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1844