Scripting Reference
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Rigidbody.sleepVelocity

var sleepVelocity : float

Description

The linear velocity, below which objects start going to sleep. (Default 0.14) range { 0, infinity }

See Rigidbody Sleeping for more information.

JavaScript 
rigidbody.sleepVelocity = 0.1;
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        rigidbody.sleepVelocity = 0.1F;
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        rigidbody.sleepVelocity = 0.1F