Contubernium, centuria and manipulus

[Cesco]

We know that the contubernium was the group of 8 soldiers who slept under the same tent. I have generaly assumed that the contubernales were all from the same centuria, but I never came across a source that states that clearly, so maybe the men of the same contubernium could have been both from the centuria prior and the centuria posterior of the same cohort. 
Of course this is just an assumption, but I wonder if some ancient source could give me some more informations about the actual composition of a contubernium.