Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) was in the state of ihram. He mounted a she-camel and it became stubborn: it would put one foot forward and pull the other back. So he said: "It is as if her rider were a twig upon a fan when she sways off with him, or a drowsy drinker." Then he said: 'Allahu akbar, Allahu akbar' (Allah is the Greatest, Allah is the Greatest).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9183