Sayyiduna Anas bin Malik (may Allah be pleased with him) says that whenever the Messenger of Allah (peace and blessings be upon him) intended a journey, he would say when he rose from his seat: 'Allahumma bika intashartu, wa ilayka tawajjahtu, wa bika i'tasamtu, anta thiqati wa raja'i. Allahumma-kfini ma ahammani wa ma la ahtammu bihi wa ma anta a'lamu bihi minni. Allahumma zawwidni al-taqwa, wa-ghfir li dhanbi, wa wajjihni ila al-khayri haythuma tawajjahtu' (O Allah, by You I set out, to You I turn, and by You I hold fast; You are my trust and my hope. O Allah, suffice me against what worries me, what I do not concern myself with, and what You know better than I. O Allah, provide me with piety as my provision, forgive me my sin, and direct me to good wherever I turn.) Then he would set out.
The common people say it like this: "bika intashartu"; but Abu Sulayman al-Khattabi (may Allah have mercy on him) used to say: The correct form is "ibtasartu", meaning: I began my journey.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10306