Samādhi Sambojjhanga…
Next, he develops Samādhi (concentration) as an element for the realization of the Four Noble Truths. Attainment of a concentration is a bojjhanga Dhamma. You know that an Arahant without a concentration cannot be formed. An Anāgāmī (non-returner) without a concentration cannot be formed. A Sakadāgāmī (Once-returner) or Sōwān (stream-winner) without a concentration cannot be formed. This is because concentration is there even in a once-returner or stream-winner up to a certain level. In general when a disciple attains the once-returner state, he develops concentration up to the fourth state of contemplation. When he becomes an Arahant he has completed the concentration.