I do not know what his source is...
http://vaticaninsider.lastampa.it/en/the-vatican/detail/articolo/22827
Here is his comment.....
As the Church strides towards the Conclave which is due to start on 11 March, the complete picture of Benedict XVI’s resignation is gradually forming. Ratzinger showed no less courage in resigning because “of the limits of old age and (…) the discernment on the exercise of responsibility that God had entrusted to him, than John Paul II who stuck it out until the end, despite his illness. Ratzinger’s decision is above all a reminder of one’s responsibilities, especially for cardinals who have the task of electing the Pope’s successor.