مناسك حج - مكارم شيرازى، ناصر - الصفحة ٥٧ - Process of Ihram
Article ٦٩- If the pilgrim cannot express these word with correct Arabic accent, it is enough that another person says them articulately for him/ her and the pilgrim recites them after that person, and if he/ she cannot pronounce correctly he/ she should say as much as he/ she can and express also its meaning as a precaution and a person who has the ability of pronouncing correctly, say the words in behalf of that pilgrim, too.
Article ٧٠- It is obligatory to say Labbayk during Ihram only once( in the way that mentioned before )and after that it is Mustahab to repeat that in different conditions as much as it is possible, it means during getting in and leaving the car, passing low and high lands, after awaking and after prayers and it is better that men recite these words with loud voice.
Article ٧١- It is obligatory that the pilgrim renounces Labbayk when he/ she sees the houses of Mecca during Ihram of Umrah al- Tamattu, and